1. Field of the Invention
The invention in general relates to miniature switches and more particularly to a switch useful in radar and other high frequency applications.
2. Description of Related Art
A variety of MEMS (microelectromechanical systems) switches are in use, or proposed for use, in radar, as well as other high frequency circuits for controlling RF signals. These MEMS switches are popular insofar as they have a relatively high off impedance and a relatively low on impedance, with a low off capacitance, leading to desirable high cutoff frequencies. Additionally, the MEMS switches have a small footprint and can operate at high RF voltages.
These MEMS switches generally have electrostatic elements which are attracted to one another upon application of a control signal. This may possibly lead to short and long term variability in the switching voltage due to, for example, uncontrolled electrostatic charging effects. In addition, the manufacturing yield and potential need for hermetic sealing of the MEMS switches are of concern.
Traditionally, many RF circuits utilize solid state switch elements for RF signal control. For example, electronic RF switches in common use include the GaAs (gallium arsenide) based PMEMT (pseudomorphic high electron mobility transistor) and the GaAs pin diode. Both of these devices can operate at high cutoff frequencies and can achieve switching rates measurable in tens of nanoseconds.
For some applications however, the GaAs PHEMT has an objectionably high resistance when closed and a relatively low cut-off frequency, for example, 600 GHz (Gigahertz). The pin diode exhibits a higher cut-off frequency of around 2 THz (Terahertz), however it, along with the GaAs PHEMT, can exhibit an objectionably high capacitance in the off state. For this reason these RF switches are often operated with a separate shunt inductor resonant with the capacitance, at the operating frequency.
This added inductor advantageously increases the impedance of the switch in the off condition, however this arrangement objectionably lowers the operating bandwidth of the overall switch device.
In an ideal switch, during operation the switch impedance would be infinite when open (Zoff), to prevent signal flow, and zero (Zon) when closed to allow signal flow without adding any undesired impedance. That is, the ideal Zoff/Zon would be equal to infinity. This however is not possible and a practical acceptable value would be on the order of 100 or 200. Most variable capacitive MEMS type and solid state switches do not achieve this high isolation ratio.
This Zoff/Zon ratio is also related to the ratio of the cutoff frequency Fcc to the operating frequency Fo. That is:
Zoff/Zon=Fcc/Foxe2x80x83xe2x80x83Eq. (1)
Basically, the higher the cutoff frequency Fcc the higher the operating frequency Fc can be while still maintaining a desired off/on impedance ratio of 100 or higher. For example, with a Fcc=1 THz, the operating frequency can be as high as 10 GHz. Devices with higher cutoff frequencies would accommodate even higher operating frequencies, particularly suited for radar and other high frequency circuits.
It is a primary object of the present invention to provide a solid state switch having a cutoff frequency in the THz range, and which is highly reliable, is reproducible, economical and has a small footprint. Additionally, the switch of the present invention can achieve a Zoff/Zon ratio of 100 or greater at 10 GHz or more. Further, the switch allows realization of cutoff frequencies as high as 5 THz as a result of its unique fabricatable geometry.
An RF switch in accordance with the present invention includes a plurality of parallel semiconductor fingers each having first and second ends, with one of the ends constituting a source region and the other end constituting a drain region. Each finger has a width no more than around 5000 xc3x85. An oxide layer is formed on the outside of the finger and a high resistance gate layer is deposited on the oxide layer. This gate layer has a sheet resistance ranging from around 100,000 to millions of ohms per square. Respective electrical contacts are in electrical communication with the gate layer and source and drain regions.